1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly, to a method of fabricating an LCD having an improved alignment characteristic with a coating-type compensation film.
2. Description of the Related Art
Liquid crystal (LC) molecule has anisotropy. The anisotropy of a LC cell having the LC molecules or a film may be changed according to distribution of the LC molecules and/or tilt angles relative to a substrate. The change of the anisotropy also changes the polarization of light relative to a viewing angle from which the LC cell is viewed. Due to unique characteristics of the LC cell, brightness and contrast ratio of a display may be different depending on a viewing angle from a top/bottom position and a left/right position.
A compensation film may be attached to compensate the anisotropy distribution due to the viewing angle of the LC cell. The compensation film is manufactured to have anisotropy distribution opposite to the anisotropy distribution of the LC cells so that a retardation difference due to the viewing angle may be removed when the compensation film is attached on the cells.
The compensation film using a high molecular film generates a phase difference of the transmitted light. The compensation film, which is extension-processed to a predetermined direction, has a birefringence due to the anisotropy of the molecules.
When an external magnetic field is applied to a twisted nematic (TN) LCD of a normally black mode, the LC molecules arrange in response to the electric field, so that light transmission occurs according to the following equation:I=IO sin 2[θ(1+u2)½], u=πR/θλ, R=Δn·d  (1)where I is intensity of transmitted light, IO is luminosity of incident light, Δn is an birefringence index, d is the thickness of an LC cell, λ is the wavelength of the transmitted light, θ is the twisted angel of twisted nematic LC, and R is a phase difference. As shown in the above equation, the phase difference has the close relationship with the viewing angle. Therefore, the phase difference may be compensated so as to improve the viewing angle.
The compensation film that compensates for the phase difference and disposed between a substrate and a polarization plate may be either uniaxial material or biaxial material. Both uniaxial material and biaxial material have an anisotropic refractive index.
FIGS. 1A to 1C illustrate an ellipse having an anisotropic refractive index of a phase-difference compensation film. Assuming that the refractive indices of x, y, and z directions in an orthogonal coordinate system are nx, ny, and nz, respectively, the uniaxial property and the biaxial property are determined depending on whether nx and ny are identical or not.
Referring to FIG. 1A, in case where the refractive indices of two directions are the same and different from the refractive index of the remaining direction in the orthogonal coordinate system, it is defined “uniaxial.” For example, in the uniaxial material, the relationship of three refractive indices are nx=ny>nz. On the contrary, referring to FIGS. 1B and 1C, in case where the refractive indices are different from one another such as nx>nz>ny and nx>ny>nz, it is defined “biaxial.”
A compensation film is generally formed of the uniaxial material having an anisotropic refractive index. Such compensation film has an arrangement that a longer axis of the ellipse is parallel or perpendicular to the surface of the film.
The compensation film is fabricated using a method of stretching a high molecular film along one axis or two-axes. The birefringence can be obtained by having the optical axis of the compensation film form an arbitrary angle with respect to the progression direction of the film. Another method of forming the compensation film is directly coating the compensation film on a substrate. This method is often referred to as a coating-type compensation film.
The coating-type compensation film is manufactured as follows. First, after an alignment film is formed on a substrate, an alignment process is performed. The alignment process allows the optical axis of the compensation film to have an arbitrary angle afterwards.
Subsequently, an LC of optical hardness is coated with a coatable retarder material on the alignment-processed alignment layer. The LC-coated substrate is irradiated with light, so that a nematic LC of optical hardness is hardened and fixed as a film. Another alignment film for aligning the LC interposed between the upper and lower substrates of the LCD is additionally formed on the coating-type compensation film.
In the related art method of fabricating the coating-type compensation film, the alignment film initially formed on the substrate is formed by printing an organic high molecular material such as polyimide and polyamide on the substrate as an alignment material and hardening the same.
The alignment process performed on the hardened alignment film applies a rubbing method of rubbing an alignment film in a predetermined direction using a rubbing cloth of a predetermined shape. As a result, grooves may be formed in a predetermined direction on the surface of the alignment film.
The above rubbing method has advantages of easy alignment and stable alignment. Further, the rubbing method is also suitable for mass production. However, the rubbing method has a problem that a rubbing defect occurs when a roller having a defective rubbing cloth attached thereon is used during the rubbing process. Because the rubbing method using the rubbing cloth requires a direct contact between the alignment film and the rubbing cloth, it may contaminate LC due to particle attachments and a non-uniformity of the alignment upon application to a large-sized LCD. Accordingly, there is a need of a fabricating method of an LCD that overcomes drawbacks of the related art fabrication method.